THE EFFECT OF OSMOTIC SHOCK AND SUBSEQUENT ADAPTATION ON THE THERMOTOLERANCE AND CELL MORPHOLOGY OF LISTERIA-MONOCYTOGENES

The relationship between the time of exposure to different levels of N aCl and the corresponding changes in thermotolerance and cell morpholo gy of Listeria monocy,togenes was investigated. The kinetics of the in crease in thermotolerance, after an osmotic upshift, showed a very rap id initial response(<2 min) followed by a more gradual increase whereb y cells, after 4 h exposure at 30 degrees C, became nearly as heat res istant as those grown for 48 h under the same conditions. Cells grown in media with 0.09 mol 1(-1) NaCl subjected to a short osmotic up-shoc k in media containing 0.5, 1.0 or 1.5 mol 1(-1) NaCl showed a 1.3, 2.5 and 8-fold increase in thermotolerance, respectively. Osmotic adaptat ion, signified by growth at the higher NaCl concentration, however, re sulted in a 2- to 3-fold additional increase in thermotolerance. An os motic down-shock caused a very rapid loss of thermotolerance (< 5 min) . Osmotic shock and adaptation experiments were also performed in minc ed beef where similar changes in thermotolerance were observed. Cell m orphology was markedly affected by the osmolarity of the growth medium . Cells grown in media containing 1.5 mol 1(-1) NaCl became up to 50 t imes longer than cells grown in media with 0.09 mol 1(-1) NaCl, but no direct link to thermotolerance could be made.